Weakness of the San Andreas Fault revealed by samples from the active fault zone

Carpenter, B. M.; Marone, Chris; Saffer, D. M.

doi:10.1038/ngeo1089

Cited by 258 publications

(226 citation statements)

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“…Drill cuttings from the country rock exhibit friction values consistent with Byerlee's law (μ = 0.40-0.66) [Tembe et al, 2006;Carpenter et al, 2009], whereas clay and serpentine grains separated from the cuttings are slightly weaker (μ = 0.30-0.50) [Morrow et al, 2007]. Experiments on cuttings from the CDZ document a low coefficient of friction (μ < 0.25), low (near-zero) rates of frictional healing, and strong localization of these mechanical properties to the creeping gouge zone [Carpenter et al, 2011]. These behaviors are correlated with the presence of magnesium-rich clays, suggesting that clay mineralogy is an important control on the strength and healing behavior of the fault, at least at these depths [e.g., Schleicher et al, 2010;Bradbury et al, 2011;Holdsworth et al, 2011;Hadizadeh et al, 2012;Richard et al, 2014].…”

Section: 1002/2015jb011963supporting

confidence: 59%

“…Experiments performed on powdered core from within and surrounding both the SDZ and CDZ show that abundant magnesium-rich clay (saponite and corrensite) localized within the faults results in low frictional strength (μ < 0.15), whereas the surrounding rock is stronger (μ = 0.3-0.6) [Carpenter et al, 2011;Lockner et al, 2011;Coble et al, 2014]. This is consistent with several studies that have documented the important role of gouge composition-and clay content in particular-on frictional properties [e.g., Lupini et al, 1981;Logan and Rauenzahn, 1987;Brown et al, 2003;Ikari et al, 2009].…”

Section: 1002/2015jb011963mentioning

confidence: 99%

“…These initiatives have recovered samples of fault rock and gouge and conducted measurements of in situ stress, fluid chemistry, and temperature at depths where earthquakes nucleate. Samples and data emerging from these drilling projects provide an unparalleled opportunity to gain new insight into absolute fault strength [e.g., Brune et al, 1969;Zoback et al, 1987;Scholz, 2000;Carpenter et al, 2012], causes of apparent fault weakness [e.g., Rice, 1992;Faulkner and Rutter, 2001;Brantut et al, 2011;Gratier et al, 2011;Lockner et al, 2011], and the laws that govern fault slip and failure [e.g., Dieterich, 1979;Marone, 1998a;Ikari et al, 2009;Carpenter et al, 2011;Sone et al, 2012].…”

Section: Introductionmentioning

confidence: 99%

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Frictional properties of the active San Andreas Fault at SAFOD: Implications for fault strength and slip behavior

2015

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We present results from a comprehensive laboratory study of the frictional strength and constitutive properties for all three active strands of the San Andreas Fault penetrated in the San Andreas Observatory at Depth (SAFOD). The SAFOD borehole penetrated the Southwest Deforming Zone (SDZ), the Central Deforming Zone (CDZ), both of which are actively creeping, and the Northeast Boundary Fault (NBF). Our results include measurements of the frictional properties of cuttings and core samples recovered at depths of ~2.7 km. We find that materials from the two actively creeping faults exhibit low frictional strengths (μ = ~0.1), velocity‐strengthening friction behavior, and near‐zero or negative rates of frictional healing. Our experimental data set shows that the center of the CDZ is the weakest section of the San Andreas Fault, with μ = ~0.10. Fault weakness is highly localized and likely caused by abundant magnesium‐rich clays. In contrast, serpentine from within the SDZ, and wall rock of both the SDZ and CDZ, exhibits velocity‐weakening friction behavior and positive healing rates, consistent with nearby repeating microearthquakes. Finally, we document higher friction coefficients (μ > 0.4) and complex rate‐dependent behavior for samples recovered across the NBF. In total, our data provide an integrated view of fault behavior for the three active fault strands encountered at SAFOD and offer a consistent explanation for observations of creep and microearthquakes along weak fault zones within a strong crust.

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Section: 1002/2015jb011963supporting

confidence: 59%

Section: 1002/2015jb011963mentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Frictional properties of the active San Andreas Fault at SAFOD: Implications for fault strength and slip behavior

2015

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“…That explanation does not seem likely, at least for crustal faults, as measurements of pore pressure within Earth's crust generally indicate hydrostatic conditions [Townend and Zoback, 2000], even in the SAF itself [Zoback et al, 2010]. Other theories appeal to anomalously low frictional resistance, from either frictionally weak clayrich materials [Morrow et al, 2000;Carpenter et al, 2011] or weak fault zone fabrics [Collettini et al, 2009] at seismogenic depths. Weak materials occur within the creeping section of the SAF and appear to be the most likely explanation for its weakness, but that cannot explain the similarly low stresses on other seismogenic parts of the SAF.…”

Section: Introductionmentioning

confidence: 99%

Additional shear resistance from fault roughness and stress levels on geometrically complex faults

2013

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[1] The majority of crustal faults host earthquakes when the ratio of average background shear stress b to effective normal stress eff is b / eff 0.6. In contrast, mature plate-boundary faults like the San Andreas Fault (SAF) operate at b / eff 0.2. Dynamic weakening, the dramatic reduction in frictional resistance at coseismic slip velocities that is commonly observed in laboratory experiments, provides a leading explanation for low stress levels on mature faults. Strongly velocity-weakening friction laws permit rupture propagation on flat faults above a critical stress level pulse / eff 0.25. Provided that dynamic weakening is not restricted to mature faults, the higher stress levels on most faults are puzzling. In this work, we present a self-consistent explanation for the relatively high stress levels on immature faults that is compatible with low coseismic frictional resistance, from dynamic weakening, for all faults. We appeal to differences in structural complexity with the premise that geometric irregularities introduce resistance to slip in addition to frictional resistance. This general idea is quantified for the special case of self-similar fractal roughness of the fault surface. Natural faults have roughness characterized by amplitude-to-wavelength ratios˛between 10 -3 and 10 -2 . Through a second-order boundary perturbation analysis of quasi-static frictionless sliding across a band-limited self-similar interface in an ideally elastic solid, we demonstrate that roughness induces an additional shear resistance to slip, or roughness drag, given by drag = 8 3˛2 G * / min , for G * = G/(1 -) with shear modulus G and Poisson's ratio , slip , and minimum roughness wavelength min . The influence of roughness drag on fault mechanics is verified through an extensive set of dynamic rupture simulations of earthquakes on strongly rate-weakening fractal faults with elastic-plastic off-fault response. The simulations suggest that fault rupture, in the form of self-healing slip pulses, becomes probable above a background stress level b pulse + drag . For the smoothest faults (˛ 10 -3 ), drag is negligible compared to frictional resistance, so that b pulse 0.25 eff . However, on rougher faults (˛ 10 -2 ), roughness drag can exceed frictional resistance. We expect that drag ultimately departs from the predicted scaling when roughness-induced stress perturbations activate pervasive off-fault inelastic deformation, such that background stress saturates at a limit ( b 0.6 eff ) determined by the finite strength of the off-fault material. We speculate that this strength, and not the much smaller dynamically weakened frictional strength, determines the stress levels at which the majority of faults operate.Citation: Fang, Z., and E. M. Dunham (2013), Additional shear resistance from fault roughness and stress levels on geometrically complex faults,

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“…The evolution of the coefficient of friction on a fault surface during and after an earthquake is time-dependent 15 . Of particular importance is the stability of phyllosilicate phases with low dynamic friction 16 , thermal expansion and the generation of physicochemical reaction products produced during slip 13 , and the presence of low-permeability mineral cements LETTER RESEARCH that enhance dynamic fluid pressurization mechanisms 17 . Temperature and fluids within fault zones are primary controls on material properties and slip-weakening mechanisms, and hence they strongly influence earthquake processes.…”

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confidence: 99%

Extreme hydrothermal conditions at an active plate-bounding fault

Sutherland

Townend

Toy

et al. 2017

Nature

108

174

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Weakness of the San Andreas Fault revealed by samples from the active fault zone

Cited by 258 publications

References 23 publications

Frictional properties of the active San Andreas Fault at SAFOD: Implications for fault strength and slip behavior

Frictional properties of the active San Andreas Fault at SAFOD: Implications for fault strength and slip behavior

Additional shear resistance from fault roughness and stress levels on geometrically complex faults

Extreme hydrothermal conditions at an active plate-bounding fault

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